1. Field of the Invention
The present invention relates generally to radio telephone apparatus for performing space diversity reception to communicate with base stations provided under a TDMA (Time Division Multiple Access) radio communication system, and is directed to an improvement in a TDMA radio telephone apparatus suitable for use as a mobile telephone equipment operative to communicate with a plurality of dispersed base stations.
2. Description of the Prior Art
There has been proposed, as one of TDMA radio communication systems, a digital cellular telephone system in which a service area is divided into a plurality of service blocks each called a cell and a single base station is located in each cell. In the digital cellular telephone system, communications between subscribers in the service area are carried out through the base stations located respectively in the cells. This means that a TDMA radio telephone apparatus used by each subscriber in the service area is operative to transmit a data signal to the base station and receives a data signal from the base station. The TDMA radio telephone apparatus is likely to be installed in a vehicle to constitute a mobile telephone equipment.
FIG. 1 shows an example of a service area provided under the digital cellular telephone system in which base stations are located. In the example shown in FIG. 1, the service area includes seven cells CA, CB, CC, CD, CE, CF and CG and base stations Sa, Sb, Sc, Sd, Se, Sf and Sg are located in the cells CA to CG, respectively. Each of the base stations Sa to Sg is connected with a MTSO (Mobile Telephone Switching Office). A TDMA radio telephone apparatus MS used by the subscriber in the cell CA, for example, is operative to select the base stations Sa located in the cell CA as a specific base station corresponding thereto so as to communicate with the base stations Sa.
The communication between each of the TDMA radio telephone apparatus and the base stations in the service area is made in accordance with the TDMA method. The TDMA radio telephone apparatus used by the subscriber, which is, for example, installed in a vehicle to constitute the mobile telephone equipment, is provided with a couple of antennas, one of which is used for both transmission and reception of data signals and one of which is selected by an antenna selecting portion in accordance with space diversity technique on the occasion of the reception of a data signal.
For the communication between each of the TDMA radio telephone apparatus and the base stations in the service area, a unit period of time called a frame period (FP), an example of which is shown in FIG. 2, is provided to be repeated. In the frame period shown in FIG. 2, the whole duration corresponds to 20 ms as indicated with 1 FP and is divided into four segmental periods including a signal transmission period (T) of 6.67 ms, an antenna selecting operation period (LM) of 1 ms, a signal reception period (R) of 6.67 ms and a base station observation period (I) of 5.66 ms.
When the TDMA radio telephone apparatus is actually used by the subscriber in the service area, a data signal from the radio telephone apparatus is transmitted, through one of the antennas provided on the TDMA radio telephone apparatus, to the specific base station which is located in the cell wherein the TDMA radio telephone apparatus is positioned, such as the base station Sa for the TDMA radio telephone apparatus MS shown in FIG. 2, in the signal transmission period contained in each frame period. In the antenna selecting operation period contained in each frame period, one of the antennas provided on the TDMA radio telephone apparatus is selected by the antenna selecting portion in accordance with space diversity technique, and in the signal reception period contained in each frame period, a data signal from the specific base station is received by the radio telephone apparatus through the antenna selected by the antenna selecting portion, so that a space diversity reception is conducted. Further, in the base station observation period contained in each frame period, a data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located, such as the base stations Cb to Cg for the radio telephone apparatus MS shown in FIG. 2, is received by the TDMA radio telephone apparatus through the antenna selected by the antenna selecting portion, so that a receiving condition of the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is observed. The data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located has a carrier frequency different from that of the data signal from the specific base station.
The result of observation of the receiving condition of the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station, which is obtained in each frame period, is transmitted from the TDMA radio telephone apparatus, together with a data signal, to the specific base station in the signal transmission period contained in the next frame period.
In the TDMA radio telephone apparatus in which the space diversity reception is performed as described above when the data signal from the specific base station or one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is received, one of the antennas provided on the TDMA radio telephone apparatus is selected by the antenna selecting portion in accordance with space diversity technique in the antenna selecting operation period contained in each frame period and therefore the data signal from the specific base station is satisfactorily received through the selected antenna in the data reception period next to the antenna selecting operation period in each frame period.
However, it is not guaranteed that the receiving condition of the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is detected under a condition in which the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is also satisfactorily received through the selected antenna in the base station observation period next to the signal reception period in each frame period. The reason for this is that the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located has the carrier frequency different from that of the data signal from the specific base station and therefore the receiving condition in which the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is received through the antenna selected in the antenna selecting operation period is almost different from the receiving condition in which the data signal from the specific base station is received through the antenna selected in the antenna selecting operation period.
Consequently, under the situation in which one of the antennas provided on the TDMA radio telephone apparatus is selected by the antenna selecting portion in accordance with space diversity technique in the antenna selecting operation period contained in each frame period and the selected antenna is used for receiving the data signals in both of the signal reception period and the base station observation period next to the signal reception period, it is feared that, although the data signal from the specific base station is satisfactorily received through the selected antenna in the data reception period next to the antenna selecting operation period, the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is unsatisfactorily received through the selected antenna to be subjected to observation of the receiving condition thereof in the base station observation period next to the signal reception period. In the case where the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is unsatisfactorily received through the selected antenna in the base station observation period, a wrong result of the observation of the receiving condition of the data signal from one of the base stations located respectively in the cells adjacent to the cell wherein the specific base station is located is obtained in the base station observation period and the wrong result thus obtained is undesirably transmitted from the TDMA radio telephone apparatus, together with a data signal, to the specific base station in the signal transmission period next to the base station observation period.